Physical Chemistry 2.2 - Reaction Feasibility - Feasability of a Reaction, Gibbs Free Energy and Total Entropy Flashcards
Spontaneous reactions happen without any ___.
input
Non-spontaneous reactions ___ happen, but they require…
can, input
Any reaction is feasible if…
ΔG is negative
Almost all ___ reactions are spontaneous in standard conditions, but they may be very ___.
exothermic, slow (eg iron rusting)
The endothermic reverse reaction ___ occurs spontaneously in ___ conditions, but it can happen in ___-___ conditions.
never, standard, non-standard
Endothermic ___ can be spontaneous in standard conditions, but ___ are not ___!
changes (eg ice melting), changes are not reactions!
Name three factors which affect the spontaneity of a reaction.
enthalpy, entropy, temperature
Gibbs free energy (ΔG°) is the energy that can be…
used to do work
What can the change in Gibbs free energy (ΔG) be used to predict?
the spontaneity (or feasibility) of a reaction
If ΔG is positive, a reaction…
If ΔG is negative, a reaction…
isn’t feasible and will never happen.
is feasible and will happen (however slow it may be)
When does a reaction become feasible?
When ΔG = 0.
In which two ways can you calculate ΔG?
- using the standard free energies of the reactants and products:
ΔG. = ∑G° (products) - ∑ΔG° (reactants) - using the change in enthalpy, the temperature and the change in entropy:
ΔG = ΔH - TΔS
* both in data booklet
For each of the following, state its name and the units it’s measured in.
1. ΔS
2. ΔH
3. ΔG
4. T
- ΔS - Entropy (measured in J/K/mol - must be converted to kJ/K/mol for calculations)
- ΔH - Enthalpy (measured in kJ/mol)
- ΔG - Gibbs Free Energy (measured in kJ/mol)
- T - Temperature (measured in Kelvin - K)
Which equation should be used to find the temperature at which a reaction becomes feasible?
T = ΔH / ΔS
(sub in G=0 to ΔG = ΔH - TΔS and rearrange for T)
Because of the whole -TΔS situation, temperature can ___ or ___ to make a reaction feasible.
A feasible reaction is any reaction where ‘ΔH - TΔS’ (aka Δ_) is _ve.
increase, decrease.
ΔG, negative
What does G equal at equilibrium?
0.
A reversible reaction will proceed spontaneously until…
ΔG = 0.
Equilibrium can be reached starting from just the ___ or ___, despite the fact that one of the reactions has a ____ ΔG.
reactants, products, positive
How does *this not violate the second law of thermodynamics?
*(Equilibrium can be reached starting from just the reactants or products, despite the fact that one of the reactions has a positive ΔG.)
because the positive ΔG value that was calculated was for the complete conversion of one mole of reactants to products, and (more importantly) the reaction mixture has a higher entropy, and therefore lower ΔG.
Since the reaction mixture of an equilibrium has a higher entropy (and therefore __er ΔG) than the reactants or products, reactions to reach equilibrium will always have a ___ ΔG.
This means they’ll be ___!
lower, negative.
Feasible
If ΔG is positive, the position of equilibrium will be more towards the ___.
If ΔG is negative, the position of equilibrium will be more towards the ___.
reactants.
products.
What is the equation for:
1. Total entropy change?
2. Entropy change of a system?
3. Entropy change of the surroundings?
- system + surroundings - ΔS(Total) = ∑ΔS (system) + ∑ΔS (surroundings)
- Products - reactants - ΔS(system) = ∑ΔS (products) - ∑ΔS (reactants)
- ΔS(surroundings) = -ΔH/T
* -ΔH/T is just the T = ΔH/ΔS equation, but rearranged for S, and inverted because it’s for the surroundings not the system (whatever happens to the system, the opposite happens to the surroundings - push and pull theory
For a feasible reaction, the ___ entropy must ___.
total, increase
ΔS (Total) must be _ve for a spontaneous reaction, but the ΔS of the system or surroundings can be _ve, as long as ΔS (Total) is still _ve.
+ve, -ve, +ve
In the equation ΔG = ΔH - TΔS, ΔS is of the ___, which means it can be _ve and the reaction can still be ___.
system, -ve, feasible
